Edmontonia was one of the largest nodosaurids, one of the two main groups of the armored ankylosaurs. Nodosaurids characteristically had a boxlike head and bony armor covering the neck, back, and upper surfaces of the tail. This armor consisted of three types of bony elements embedded in the skin. The largest were pronounced spikes, on the shoulders and forming two rows running along the sides of the animal. Shieldlike scutes of varying sizes were arranged in several rows, running lengthwise from the back of the neck to the tip of the tail. In between the scutes and spikes were thousands of small, pea-sized ossicles. Together the spikes, scutes, and ossicles formed an impenetrable shield against the attacks of predators. Even the head had a set of interlocking bony plates over the upper surfaces to protect the brain, eyes and nose.

The huge spikes on the shoulders gave Edmontonia an offensive weapon. By tucking its bony head below them, Edmontonia could drive these spikes forward into an attacker with potentially lethal effect. As with most nodosaurids, Edmontonia's belly was unprotected by armor and would have been vulnerable to attack if the animal were flipped over. To prevent this from happening, Edmontonia was very low-slung with relatively short, stumpy legs spread wide by broad hips and shoulder girdles. Edmontonia was built rather like a huge coffee table!

Edmontonia's boxy head had a cropping beak at the front of the mouth and rows of small, serrated, triangular teeth in the cheeks. Wear on these cheek teeth indicates that Edmontonia snipped its food into tiny pieces before swallowing it. Edmontonia was one of the last of the nodosaurid dinosaurs. Other nodosaurids are known from around the world in older rocks dating back to the latest Jurassic.